Different levels of surface stain due to the glass weathering, and the stain appears as haze and rainbow phenomenon. Slight haze can be removed by washing(acid washing) to recover the surface quality, but when glass becomes a heavy stain, the surface corroded had to be removed by polishing them to meet quality requirements.
Comparing other materials, the cost of interleaving powder is 1/6 of interleaving paper but reach the same efficiency, that's why interleaving powder is more and more popular in the glass industry.